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RT9594A

MOSFET Integrated Smart Photoflash Capacitor Charger with IGBT Driver

Richtek Technology 

Application Information

The RT9594A/B integrate a constant peak current controller

for charging photoflash capacitor and an IGBT driver for

igniting flash tube.

The photoflash capacitor charger uses constant primary

peak current and SW falling control to efficiently charge

the photoflash capacitor. Pulling the CHARGE pin high

will initiate the charging cycle. During MOS on period,

the primary current ramps up linearly according to VBAT

and primary inductance. A resistor connecting from CS

pin to GND determines the primary peak current.

During the MOS off period, the energy stored in the flyback

transformer is boosted to the output capacitor. The

secondary current decreases linearly at a rate determined

by the secondary inductance and the output voltage

(neglecting the voltage drop of the diode). The SW pin

monitors the secondary current. When the secondary

current drops to 0A, SW voltage falls then MOS on period

starts again. The charging cycle repeats itself and charges

the output capacitor.

The output voltage is sensed by a voltage divider connecting

to the anode of the rectifying diode. When the output

voltage reaches the desired voltage set by the resistor

divider, the charging block will be disabled and stop

charging. Then STAT pin will be pulled low to indicate the

complete charging. The voltage-sensing path will be cut

off when charging completed to minimize the output voltage

decay. Both the CHARGE and STAT pins can be easily

interfaced to a microprocessor in a digital system.

Transformer

The flyback transformer should be appropriately designed

to ensure effective and efficient operation.

1. Turns Ratio

The turns ratio of transformer (N) should be high enough

so that the absolute maximum voltage rating for the

internal N-MOSFET drain to source voltage is not

exceeded. Choose the minimum turns ratio according to

the following formula :

NMIN

≥

VOUT

45 − VBAT

Where :

VOUT : Target Output Voltage

VBAT : Battery Voltage

DS9594A/B-02 April 2011

RT9594A/B

2. Primary Inductance

For each switching cycle, energy transferred to the output

capacitor is proportional to the primary inductance for a

constant primary current. The higher the primary

inductance is, the higher the charging efficiency will be.

Besides, to ensure enough off time for output voltage

sensing, the primary inductance should be high enough

according to the following formula :

L PRI

≥

400 × 10−9 × VOUT

N × IPK−PRI

VOUT : Target Output Voltage

N : Transformer turns ratio

IPK-PRI : Primary peak current

3. Leakage Inductance

The leakage inductance of the transformer results in the

first spike voltage when N-MOSFET turns off. The spike

voltage is proportional to the leakage inductance and

inductor peak current. The spike voltage must not exceed

the dynamic rating of the N-MOSFET drain to source

voltage (50V).

4. Transformer Secondary Capacitance

Any capacitance on the secondary can severely affect the

efficiency. A small secondary capacitance is multiplied

by N2 when reflected to the primary will become large.

This capacitance forms a resonant circuit with the primary

leakage inductance of the transformer. Therefore, both the

primary leakage inductance and secondary side

capacitance should be minimized.

Rectifying Diode

The rectifying diode should be with short reverse recovery

time (small parasitic capacitance). Large parasitic

capacitance increases switching loss and lowers charging

efficiency. In addition, the peak reverse voltage and peak

current of the diode should be sufficient.

The peak reverse voltage of the diode is approximately :

VPK−R ≈ VOUT + (N × VBAT)

The peak current of the diode equal primary peak current

divide transformer turn ratio as the following equation :

IPK −SEC

=

IPK −PRI

N

Where : N is transformer turns ratio.

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